Surface Quantitative Precipitation Estimates (SQUIRE) from the X-Band Precipitation Radar during the Surface Atmosphere Integrated Field Laboratory (SAIL) Experiment

The U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Second Mobile Facility (AMF2) was stationed in Crested Butte, Colorado from September 2021 to June 2023 during the Surface Atmosphere Integrated Field Laboratory (SAIL) experiment. One of SAIL’s primary scientific objectives is to characterize the spatial distribution of orographic and convective precipitation processes in the Upper Colorado River Basin (UCRB) on diurnal to seasonal time-scales and how those processes interact with the large-scale circulation. Despite being the most hydrologically significant watershed in North America, observations of snowfall accumulation are lacking in UCRB, hence reliable quantitative precipitation estimates (QPE) is vital for water resource assessment, validation of hydrological modeling, and understanding of precipitation processes in complex terrain.Therefore, to provide precipitation estimates useful for this objective, we have developed the Surface Quantitative Precipitation Estimate (SQUIRE) product from the ARM-supported Colorado State University (CSU) X-band Precipitation Radar deployed during SAIL for winters 2021-2022 and 2022-2023. The CSU X-band radar was co-located with instruments measuring meteorological characteristics and precipitation accumulation at the AMF2 and at a long term measurement site in Gothic, CO.

To create these QPE products, we first applied correction algorithms from the Corrected Moments in Antenna Coordinates (CMAC) package on the CSU X-band radar data to remove clutter, calibrate the radar observations, correct for attenuation of the radar beam, and flag blocked radar beams. We then calculated snowfall rate S from the CSU X-band radar using an ensemble approach by calculating snowfall rate using various empirical relationships between radar reflectivity Z, differential reflectivity ZDR, and specific differential phase KDP and S developed in the literature for both winters. We compare the radar-estimated daily snow water equivalent (SWE) against daily SWE from both the Pluvio rain gauge and Gothic measurements for each member of the ensemble in order to determine which provides the best estimate of S from the X-band radar data. The results show that the Wolf and Snider (2012) estimates of SWE perform best when daily SWE < 2 mm. For daily SWE > 2 mm, the WSR-88D Intermountain West relationship performs better, being within 60% of the Gothic weather measured SWE.